1. Field of the Invention
The invention relates to an abnormality detection apparatus and an abnormality detection method for an air/fuel ratio sensor.
2. Description of the Related Art
An internal combustion engine for a motor vehicle or the like is provided with an air/fuel ratio sensor that outputs a signal that corresponds to the air/fuel ratio of the internal combustion engine on the basis of the oxygen concentration in exhaust gas. On the basis of the output from the air/fuel ratio sensor, the amount of fuel injected into the engine is corrected to so that the air/fuel ratio of the engine becomes equal to a stoichiometric air/fuel ratio. By controlling the air/fuel ratio of the internal combustion engine to the stoichiometric air/fuel ratio through the correction of the amount of fuel injection, good performance of exhaust purification of an exhaust purification catalyst provided in an exhaust system of the engine is maintained so that the exhaust emission of the internal combustion engine can be bettered.
With the foregoing internal combustion engine, there is a risk that abnormality of the air/fuel ratio sensor, such as degradation thereof or the like, may influence the exhaust emission. Therefore, in order to prevent such influence, the engine is provided with an abnormality detection apparatus that determines the presence/absence of abnormality of the air/fuel ratio sensor. A known abnormality detection apparatus for an air/fuel ratio sensor determines the presence/absence of abnormality of the air/fuel ratio sensor by the following procedure “1” to “3” as shown in, for example, Japanese Patent Application Publication No. 2004-225684 (JP-A-2004-225684). Firstly, as the process “1” in the procedure, an active air/fuel ratio control in which the air/fuel ratio of the internal combustion engine is periodically fluctuated between a rich state and a lean state is performed. Next, as the process “2”, a parameter that corresponds to the responsiveness of the output of the air/fuel ratio sensor is found on the basis of the output of the sensor during the active air/fuel ratio control, and the parameter is acquired as data for detecting abnormality. Finally, as the process “3”, the presence/absence of abnormality of the air/fuel ratio sensor is determined on the basis of comparison between the acquired data and an abnormality criterion value.
By the way, in recent years, the requirement for betterment of exhaust emission of the internal combustion engine has become severer. Therefore, it is considered that in order to determine that an air/fuel ratio sensor that does not meet the requirement is abnormal, the abnormality criterion value used in the process “3” is shifted toward the side of normality and therefore the determination as to the presence/absence of abnormality of the air/fuel ratio sensor is performed more severely so that it is more likely to be determined that the air/fuel ratio sensor has abnormality.
However, if in the process “3”, the foregoing determination as to the presence/absence of abnormality is made severer so that it is more likely to be determined that the air/fuel ratio sensor has abnormality, the difference between the output of the air/fuel ratio sensor during normality thereof and the output of the air/fuel ratio sensor during abnormality thereof becomes small, so that the responsiveness parameter found in the process “2” less clearly represents a difference made by the presence/absence of abnormality of the air/fuel ratio sensor. In particular, during the state of small amount of intake air of the internal combustion engine, since the exhaust gas pressure of the internal combustion engine (that corresponds to the amount of flow of exhaust gas) becomes low so that the influence caused by abnormality of the air/fuel ratio sensor, such as degradation thereof or the like, does not clearly appear in the output of the air/fuel ratio sensor, the foregoing tendency of the responsiveness parameter representing less clearly the difference made by the presence/absence of abnormality of the air/fuel ratio sensor becomes conspicuous. Furthermore, when the motor vehicle is accelerating or decelerating during the small-amount-of-intake-air state of the internal combustion engine, the responsiveness parameter greatly fluctuates due to the response delay of various appliances of the internal combustion engine, so that there is high possibility that data acquired in the process “2” will have a value that makes it hard to determine the presence/absence of abnormality of the air/fuel ratio sensor.
If the responsiveness parameter found in the process “2” less clearly represents a difference between the presence and the absence of abnormality of the air/fuel ratio sensor, it becomes difficult to accurately determine the presence/absence of abnormality of the air/fuel ratio sensor in the process “3”.